Prior art signal detectors exhibit a number of drawbacks. They are slow to act, have high false alarm rates, and miss some signals at relatively high signal-to-noise ratios. Many signal detectors compare the energy of the signal with a threshold. The signal energy is measured, approximately, with an envelope detector, or more directly using the signal itself, but for such a technique to be successful, the threshold level must be known. If the noise level changes, the performance of the detector can be degraded unless the threshold or preamplifier gain is changed to accommodate the changed circumstances. One solution to the noise change affect is to include a noise-riding threshold in the signal detector. This is accomplished by basing the threshold on a noise level that was measured some short time in the past.
Certain signals such as modem signals have a constant energy and will cause a threshold to adjust and the signal to drop below the threshold in a time comparable to the time constant of the change of the threshold. Such signals will be lost after a short time in a noise-riding threshold detector.
FM radio systems employ a detection scheme that captures a signal for detection by a "hard limiter". A hard limiter exhibits a constant power output. If a narrow band signal is received in the bandwidth of a hard limiter, most of the energy of the output will be at the frequency of the narrow band signal. The result is that the energy away from the frequency of the signal is suppressed by the "capture" effect. By providing filters that measure the energy away from the frequencies where signals are expected, suppression of hard limiter noise can be achieved. Hard limiter detectors, however, exhibit limited performance and lack of applicability to digital signals. When the time constant of the detector is short, there tends also, to be too many false alarms.
Other signal detectors employ non-parametric statistical approaches. One approach is to collect a set of samples from an input signal. The next step adjusts the signal amplitudes to have a sample mean of absolute signal values that equals a constant value (e.g., 1.0). The set of numbers that results is tested for a match to a particular distribution. One test is to rank order the samples, (i.e., arrange them in the order of their amplitudes) and then to check the difference between the 10% and 90% values. If this value changes significantly from a value when there is only noise present, a signal has been detected. This approach is difficult to implement as the process of rank-ordering a collection of samples is time consuming. If the signal is only checked at intermittent intervals, the sorting process, while slow, enables a reliable detection scheme. However, if the signal is subject to rapid changes of state, the detection procedure deteriorates rapidly.
A variety of additional signal detection schemes are described in the following prior art. U.S. Pat. No. 4,052,568 to Jankowski, entitled "Digital Voice Switch" describes the use of a set of adaptive thresholds to detect a voice signal. A plurality of thresholds are used to enable reliable detection in a short time. U.S. Pat. No. 4,667,065 to Bangetter, entitled "Apparatus and Methods For Electrical Signal Discrimination" analyzes a pattern of threshold crossings of a signal to determine whether the signal is a voice signal or some other signal with a more periodic nature. Noise is excluded by setting the basic detection threshold so that the noise does not cross the threshold. The Bangerter system primarily discriminates voice signals from other signals with the noise discrimination occurring as a result of a threshold setting.
U.S. Pat. No. 4,682,361 to Selbach et al., entitled "Method of Recognizing Speech Pauses" employs a Fourier transform to form the spectrum of a received signal. The spectrum is used to estimate signal power and noise power. When the signal power is above a noise power threshold, a signal is declared to be present.
U.S. Pat. No. 4,696,039 to Doddington, entitled "Speech Analysis/Synthesis System with Silence Suppression" estimates the level of peaks and valleys of a signal envelope and declares a signal to be present when a peak is sufficiently larger than a valley. Envelope peaks are determined by a fast rise, slow fall signal detector, and valleys are estimated by employing a slow rise, fast fall signal detector. The system described by Doddington is essentially an adaptive threshold detector.
U.S. Pat. No. 4,829,578 to Roberts, entitled "Speech Detection and Recognition Apparatus For Use With Background Noise of Varying Levels" estimates the noise level of a signal by using an adaption technique during the period before a "start of speech" and after an "end of speech". When the amplitude crosses a level based on the estimate of the noise level, the voice signal is declared to be present.
U.S. Pat. No. 4,860,359 to Eicher, entitled "Method Of Voice Operated Transmit Control" describes a computer program that performs a number of control operations involved with the operation of a push-to-talk transceiver. A voice signal detection scheme employs a set of four filters. The amplitudes of signals from the filters are compared to detect a received voice signal.
U.S. Pat. No. 4,920,568 to Kamiya et al., entitled "Method of Distinguishing Voice From Noise" is a system for detecting voiced sounds in noise. The system employs the power in the signal over an interval and the values of linear predictive filter coefficients to detect the voice signal. Whenever both the power and the sum of the filter coefficients is larger than their respective thresholds, the signal is likely to be a voice signal.
U.S. Pat. No. 4,926,484 to Nakano, entitled "Circuit For Determining That an Audio Signal is Either Speech or Non-Speech", detects a voice signal by making use of the fact that the voice signal has gaps. The gain of a detector preceding a detection threshold is adjusted up or down (effectively adjusting the threshold) in dependence on the relative amount of time that the incoming signal amplitude spends above and below a threshold. If the signal spends too much time below the threshold, the gain is increased. If the signal spends too much time above the threshold, the gain is reduced.
U.S. Pat. No. 4,926,488 to Nadas et al, entitled "Normalization of Speech by Adaptive Labeling" employs a vector quantizing scheme, with vectors being modified slowly in an adaptive manner. The incoming signal is detected only by the amount of energy within the signal within a time period.
U.S. Pat. No. 4,982,341 to Laurent, entitled "Method and Device for the Detection of Local Signals" teaches the use of the ratio of the power in a signal and the power in a high pass filtered version of the signal to detect a voice signal. Voiced signals tend to have the power in the low frequencies and the relative power at high frequencies is small. When the power of the two signals is approximately the same, the signal is assumed to be noise.
The procedures described in the above cited prior art are, in many instances, adaptive and time consuming in their performance. Of those that teach spectrum analysis, the use of the spectrum is employed to measure the energy in the signal. Generally, they also check to see if a particular kind of signal is present in the input.
Accordingly, it is an object of this invention to provide a signal detector that detects the presence of noise rather than an information signal.
It is another object of this invention to provide a signal detector that is non-adaptive and, as a result, exhibits extreme rapidity in its detection capability.
It is yet another object of this invention to provide a signal detector that provides a plurality of independent indicators of the presence of a noise signal, so as to achieve a low false alarm rate.